How can I add a realistic metallic texture to this node setup?
To start off, download the weighted normals add-on: https://blenderartists.org/uploads/default/original/4X/b/b/6/bb60fd3ddf118fca056b6840acae494f6e57a06b.zip

To give credit where it is due, I obtained everything I use in this demonstration from here: https://pr0f3550rz00m.deviantart.com/art/Thanos-Fortnite-3D-Model-744104114

Now, for each individual mesh piece: (make sure you're in object mode) Select the mesh, press SPACE and search for weighted normals and hit ENTER. Then, with the mesh still selected press SPACE again and search for Clear Custom Split Normals Data and then hit ENTER 

Note do not apply the split normals to his face, as it will cause hard edges on an organic surface, which we do not want. After applying that to mesh, we can begin the node setup!

Also you'll need to remove all of the parts which relate to his skin, as they should not be treated as hard surface objects either.

For point of reference, the metalness and specularity maps contain information as follows:

R = Ambient Occlusion 

G = Unknown

B = Roughness/ Metalness/ Emission

A = No alpha channels are included

Helmet and Torso Node Setup



This is the general setup for the helmet and the torso piece, however; the gauntlet and face have an extra part to them as they both seem to contain a part that emits light. 

Make sure that textures not affecting the colour are set to non-colour data



Face Node Setup





You could just use the original diffuse map to control the emission value, I just happened to make a duplicate of it in this example.

Gauntlet Node Setup





This time I just use the original diffuse map for the colour input. 

Just in case it isn't clear from the image, the blue channel from the metallic map is controlling the factorial of the mix node.



And here is an example of the final result: 



Hope this helps.